A. Field of the Invention
This invention relates to a color cathode ray tube apparatus and, more particularly, to a color cathode ray tube apparatus having an electron gun assembly provided with means for modulating a scanning velocity of electron beams for clarifying an image projected on a screen.
B. Description of the Prior Art
A system for modulating a scanning velocity of electron beams is known as one of the methods of producing a distinct image in a color cathode ray tube. The scanning velocity modulating systems are generally classified as electromagnetic deflecting systems and electrostatic deflecting systems. The basic principle of both systems varies the scanning velocity of electron beams on required image areas to clarify the image projected on a screen. A system for electrostatically deflecting electron beams in an electron gun assembly is disclosed as a method for modulating the scanning velocity of the electron beams in Japanese Patent Publication No. 48423/1980. A fourth grid as a focusing electrode in a unipotential type electron gun assembly disclosed in the official gazette is split, and a signal for modulating the scanning velocity in response to a luminance signal of a color video signal is supplied to the split electrodes.
Since a high voltage, such as 5 kV is applied to the focusing electrode in this system it is expensive to manufacture a circuit for modulating the high voltage. Also, since a potential difference between a high voltage electrode, or a third electrode, and an accelerating electrode, or a second electrode, is large, a problem arises in which a discharge is generated between the electrodes.
It is generally necessary to further raise the applied focusing voltage, or increase the length of a focusing electrode in an electron gun assembly to improve the focusing efficiency of the electron beam. However, if the focusing electrode is lengthened in the conventional electron gun, there are problems in that its deflecting sensitivity is deteriorated and the design of the circuit becomes difficult because of the higher modulating voltage. The voltage of the focusing electrode can be designed to be low, but, in this case, the magnification of its lens increases, and another problem occurs in which the diameter of the beam spot increases on a screen.
Since a unipotential type electron gun assembly is used in the conventional color cathode ray tube apparatus, the traveling velocity of the electron beams in the gun assembly is fast, and it is difficult to sufficiently accelerate the scanning velocity of the electron beams by scanning velocity modulating means. In addition, incident angles of three electron beams to a gap of the focusing electrode are not equal, and when electron beam scanning velocity modulating means is provided at the focusing electrode, the deflecting sensitivities of the beams are different, causing a misconvergence.
A color cathode ray tube apparatus in which means for modulating the scanning velocity of electron beams is provided at the focusing electrode of a unipotential type electron gun is difficult to manufacture for practical use, due to the above-mentioned problems.